In a cell assembly configured by combining a plurality of secondary cell cells (called a cell module or modular cell), it is important to control cell balance, that is, uniformity of cell voltages of the respective cells (single cells). When a cell voltage of a particular cell is excessively higher or lower than the cell voltages of other cells, the particular cell is sometimes over-charged or over-discharged, and accordingly deterioration of the particular cell is rapidly progressed. Especially, in a cell assembly configured by combining a plurality of lithium ion secondary cells, it is highly required to prevent the over-charge and over-discharge, and it is important to maintain the cell balance.
As shown in FIGS. 1A to 1C, as an operation method of controlling the cell balance (hereinafter referred to as a “cell balance control”), a method is known of discharging from a cell charged to a higher cell voltage (a discharging method). In this method, the cell voltages of the respective cells of the cell assembly are detected, and the minimum cell voltage of them are found. When the cell voltage of a particular one of the cells (not limited to one) is higher than the minimum cell voltage, exceeding a permissible range, the cell voltage is lowered by discharging from the particular cell.
As shown in FIGS. 2A to 2C, in another method of the cell balance control, a cell having a lower cell voltage is charged (a charging method). In this method, the cell voltages of the respective cells of the cell assembly are detected, and the maximum cell voltage of them is found. When the cell voltage of a particular one of the cells is lower than a maximum cell voltage, exceeding a permissible range, the cell voltage is increased by charging the particular cell.
However, the above-mentioned two methods both have a problem of a large energy loss. In the discharging method, when the cell voltages of a few cells are high (in a case of FIG. 1A), the cell balance control is carried out by discharging from the few cells. Accordingly, the cell balance can be controlled in a little energy loss. However, when the cell voltages of the few cells are low (in cases of FIGS. 1B and 1C), discharging is carried out from many cells other than the few cells. Thus, much energy is wasted for the cell balance control.
Since the charging is associated with energy loss at least, the same situation is applied to the charging method. In a case where the cell voltages of the few cells are low (in a case of FIG. 2A), the cell balance control is carried out by charging the few cells. Accordingly, the cell balance can be controlled in a little energy loss. However, when the cell voltages of the few cells are high (in cases of FIG. 2B and FIG. 2C), many cells other than the few cells are charged. Accordingly, the energy loss in the charging is large.
In conjunction with this, in JP H06-253463A, a circuit of carrying out the cell balance control through charging; and a circuit of carrying out the cell balance control through discharging are disclosed. It should be noted that it is not disclosed and suggested that a circuit of carrying out the cell balance control through the charging; and a circuit of carrying out the cell balance control through the discharging are prepared for one cell assembly.
From such a viewpoint, provision of a technique of reducing energy loss in a cell balance control is desired.